1. Field of the Invention
This invention relates to an X-ray image pickup tube for converting X-ray images into electric signals in an X-ray television system for use in medical diagnosis or non-destructive inspection of materials.
2. Description of the Related Art
In a conventional X-ray television system, an X-ray image intensifier and a television image pickup tube are combined to convert X-ray images into electric signals. That is, X rays enter the image intensifier where the incident X rays are converted into visible rays by a converting film such as of CsI. Thereafter electrons are released from a photoconductive film and, while being multiplied, passed to an output fluorescent film to be converted into visible light. Then, a visible light image is outputted from this output fluorescent film. The pickup tube is optically coupled to an output plane of the image intensifier. The visible light image is projected through an optical lens or the like to an image pickup plane of the image pickup tube. Consequently, electric charges corresponding to the incident light accumulate on the pickup plane, which are scanned and read by an electron beam to output electric signals.
Also known is an X-ray television system employing an X-ray image pickup tube such as an X-ray HARP (High-gain Avalanche Rushing amorphous Photoconductor) tube to convert X rays directly into electrons. This X-ray image pickup tube includes an X ray to electricity conversion film formed of a material such as amorphous selenium which is sensitive to X-ray regions, and which is the photoconductive conversion film used in the ordinary visible light image pickup tube. That is, this X-ray image pickup tube does not use the X ray to visible light conversion film such as of CsI used in the image intensifier, but instead uses an amorphous selenium film for converting X rays directly into electric charges, thereby to obtain electric signals amplified by an avalanche effect.
However, with the conventional X-ray television system combining the X-ray image intensifier and television image pickup tube, an X-ray image is converted into final electric image signals through numerous conversion steps as noted above; converting X rays to visible rays, then to electrons, to visible rays again which are passed through optics, to visible rays yet again, and to electric signals. Such a process tends to provide a poor efficiency of conversion, and inevitably causes a final image to have a low signal to noise (S/N) ratio. In addition, the combination of the image intensifier and image pickup tube has a drawback of complicating and enlarging the apparatus. With the X-ray image pickup tube such as an X-ray HARP tube having an amorphous selenium film for converting X rays directly into electric charges, the amorphous selenium film, which has a high X-ray transmittance, must be formed as thick as 500 .mu.m or thereabout in order to increase the efficiency of conversion. This is hardly practicable since an extra-high voltage is required to bring about the avalanche effect. Moreover, it is difficult from the manufacturing point of view to form the amorphous selenium film thick and uniform over a wide range. It is thus difficult to provide this type of X-ray image pickup tube with a large aperture.